Many oil and gas wells must be provided with artificial lift in order to extract the hydrocarbons in an effective manner, otherwise the relatively low natural reservoir pressure (particularly in the middle and latter years of some wells) is not sufficient to flow the well. Conventionally, the artificial lift can be provided by a variety of methods including injection of CO2 into the well to force the hydrocarbons up to the surface and by providing downhole pumps to suck in the hydrocarbons and pump them up production tubing to the surface. An Electrical Submersible Pump (ESP) is a form of artificial lift pump designed to draw fluid from a well in the absence of pressure to suit the production rate required. Typically ESPs, in the oilfield, have been run as single units on the end of the production tubing. A power cable, attached to the electrical motor unit of the ESP extends to the surface of the well alongside the production tubing and terminates at the wellhead.
The power cable will often need to be fed through a packer (a downhole barrier adapted to seal the annular gap between the production tubing and the casing) prior to extending to the surface of the well where the power cable also needs to be fed through the wellhead. At both of these junctions, the power cable usually has to be deployed with an electrical penetrator which seals the cable into the wellhead and packer. It should be noted however that not all ESP wells use packers but all require wellheads and such a typical/conventional configuration of a well having an ESP deployed therein is shown in FIG. 1.
In more recent years, it has become more customary for an operator to want to use a dual ESP configuration, where one ESP is run on top of the other, with a spacing therebetween. This configuration allows one ESP unit to be operated or run to the end of its life and then the second ESP unit is switched on. The benefits of dual ESP systems are considerable in terms of saved workover (well completion replacement), costs and avoidance of oil well downtime.
Conventional dual ESP configurations require a dedicated power cable from each of the dual ESPs to the surface of the well and therefore two power cables are required from the ESP's to the surface.
The power cable feed for the lower ESP motor extends from a plug-in connection at the lower ESP motor, up beyond the upper ESP and is joined by the power cable feed for the upper ESP. From there, both cables extend to the surface of the well and such a typical/conventional configuration of a well having a dual ESP system deployed therein is shown in FIG. 2.
In wells where the power cable has to pass through a packer as well as through the wellhead, special electrical “penetrators” (units which seal the power cable into a steel body) are required.
Dual ESP systems therefore require two penetrators, both for the packer and for the wellheads. Unfortunately, standard wellheads and packers are manufactured with only a single penetrator and cannot be modified to accept twin penetrators. Accordingly, packers and wellheads have to be specially manufactured to suit twin penetrators.
Accordingly, for new wells, packers and wellheads can be specially ordered to accommodate the twin penetrator requirement. However, existing wells would require a conversion and this leads to significant costs due to the large variety of wellhead types and the engineering required. Furthermore, the existing customer owned and very expensive wellheads and packers would therefore be scrapped.
This extra (significant) cost plus the associated lead time in obtaining such new and special wellheads currently makes conversion to dual ESPs non-viable for many existing wells or at least, presents a barrier to conversion to duals ESP systems.
It would therefore be desirable if the existing wellhead (and packer if required) can be utilised; if this was the case then conversion to dual ESPs becomes more viable and presents a significant opportunity to improve ESP viability in all manner of wells.